To investigate intraepithelial extension of CIS, thirty-two cases of carcinoma in situ (CIS) of the uterine cervix in the period of sexual maturity were examined histopathologically and compared with ten cases of microinvasive carcinoma of the uterine cervix (restricted to Japanese criteria). The results were summarized as follows. 1) A positive correlation was found between the circular spreads and longitudinal ones in the lesion in CIS cases. 2) A positive correlation was found between circular spread and the depth of gland involvement in the lesion in CIS cases. 3) Circular spread of the lesion in microinvasive carcinoma cases was significantly more extended than that in CIS cases. These results suggest that CIS in the period of sexual maturity is intraepithelially extensive and that there is a high risk possibility of invading stroma when circular spread of the CIS lesion is greater than one fourth of the whole circle of the uterine cervix.